1. Field of the Invention
The present invention relates to a method of diffusing boron into semiconductor wafers with oxidation induced stacking faults kept to an absolute minimum.
2. Description of the Prior Art
Boron nitrides (BN) and polyboron films (PBF) are known as boron sources for use in the boron diffusion of transistor base regions.
In the case where a BN source is employed, a semiconductor wafer is disposed on boats in contact at both sides of the boron source, followed by heating in a diffusion furnace. Boron oxide (B.sub.2 O.sub.3), generated with heat on the surface of BN, is allowed to fly over the wafer so that glass and diffusion layers of boron, both layers of high concentrations and uniform thicknesses, are formed on the wafer. The boron glass layer is then removed by etching, whereas the boron diffusion layer is oxidized again in the furnace at a relatively low temperature of 700.degree. C. after which the resulting contamination layer is etched from the boron diffusion layer. Subsequently, a boron diffusion step commonly termed a drive-in step is carried out during the heating in the furnace.
The above mode of boron diffusion with a BN source is not wholly satisfactory as, due to B.sub.2 O.sub.3 on BN being necessarily subject to flying over the wafer and hence likely to be affected by a gaseous stream in the furnace, sheet resistance is variable or irregular and difficult to control. As another but serious problem, this prior boron diffusion technique is susceptible to oxidation induced stacking faults (OSF) as many as 100 pieces/cm.sup.2 or greater. OSF densities of not more than 20 pieces/cm.sup.2 are regarded as critical and acceptable in the art.
PBF permits direct application or coating over a semiconductor wafer, forming B.sub.2 O.sub.3 and boron glasses of uniform thicknesses with uniform magnitudes of sheet resistance. Moreover, it is by nature easy to handle. PBF has been reputed as a good boron source as taught for instance in Japanese Patent Publication No. 1-60932 and Japanese Patent Laid-open Publication No. 60-92610. This film leaves, despite those advantages, the problem that it involves OSF in a density of more than 100 pieces/cm.sup.2.
The foregoing drawbacks of the prior art have lent an impetus to the provision of means which will enable OSF densities to be reduced in the boron diffusion of semiconductor wafers.